The present invention relates to a flame-retardant resin composition, which has a high flame retardancy equivalent to that of a polyvinyl chloride (PVC) composition and can be used as a coating material of electrical wires, cables and analogues thereof, and which can be separated from a PVC composition as a result of having a smaller specific gravity than that of the PVC compound, and which is free from halogen and can be incinerated.
PVC compounds have widely been used as electrical wire coating materials, tubes, tapes, packing materials and building materials because of their good electrical insulating properties and self-extinguishing flame retardancy.
However, since PVC compounds contains chlorine (Cl) as a halogen, corrosive gases such as HCl and a toxic gases such as dioxin are evolved on combustion. Therefore, there is the problem that wastes of various PVC products cannot be incinerated and, at present, PVC products are buried. However, since a Pb-based stabilizer is used as an additive in PVC compounds, the stabilizer is eluted into the soil, thereby making it difficult to treat PVC products as industrial waste.
When halogen-free polyethylene (PE) and polypropylene (PP) are used as resin compositions in place of PVC, they can be incinerated because no harmful gas is evolved on combustion. However, these halogen-free resin compositions have the drawbacks that the flame retardancy is inferior to PVC. Comparing the oxygen indexes (OI) of the compounds as a measure for evaluation of the resin composition, the OI of PVC is from 23 to 40, whereas the OI of PE and PP is inferior, roughly 17 to 19.
To impart the flame retardancy to a halogen-free resin composition such as PE and PP, a hydrated inorganic compound such as Mg(OH)2 is added. However, the specific gravity of the hydrated inorganic compound is large and 120 parts by weight or more of the hydrated inorganic compound must be added to obtain the flame retardancy equivalent to that of PVC, so that the specific gravity of the flame-retardant resin composition obtained by adding the hydrated inorganic compound is about equivalent to that of PVC. Therefore, there is the problem that such a plastic cannot be separated using a difference in specific gravity by using water as a standard liquid.
xe2x80x9cThe recovered plastic is put into water flowing uniformly and then separated due to a difference in specific gravity in order to recycle or remove PVCs, which evolve dioxin, before incineration. As shown in FIG. 1, reference numeral 1 denotes olefin plastic particles such as PP and PE, which float on water because their specific gravity (about 0.9 g/cm3) is lower than that of water, whereas PVCs, denoted by the reference numeral 2 in FIG. 1, have a higher specific gravity (about 1.3 g/cm3) than that of water and sink in water. Styrene plastics denoted by the reference numeral 3, also sink in water because their specific gravity is about 1.2 g/cm3. Since there are small differences in their densities between the PVCs and styrene plastics, PVCs and styrene plastics can be separated by flowing water as a uniform current, so that these plastics collect at different locations. Accordingly, the above three kinds of plastic can be separated by making use of these characteristics. However, there is the drawback that the above mentioned halogen-free flame-retardant resin composition and PVC cannot be separated because their specific gravities are about identical.xe2x80x9d
An object of the present invention is to provide a resin composition which has a flame retardancy equivalent to that of PVC and evolves no harmful gas on incineration, and which can be separated from PVC due to a difference in specific gravity.
The halogen-free flame-retardant resin composition of the present invention is prepared by adding not less than 5 parts by weight and less than 50 parts by weight of a hydrated inorganic compound and not less than 1 part by weight and not more than 55 parts by weight of an auxiliary flame retardant having an action of forming a char-like thermally insulating layer on combustion and/or an action of foaming at high temperature thereby to entrap water vapor in a polyolefine resin so that the resulting flame-retardant resin composition has a specific gravity of 1.14 or less and an oxygen index of 24 or more.
According to the present invention, a preferable flame retardancy and low specific gravity can be attained at the same time by using the hydrated inorganic compound in combination with the auxiliary flame retardant having the above actions to obtain a synergistic effect. When the specific gravity of the halogen-free frame-retardant resin composition is 1.14 or less, it is possible to fractionate it from PVC due to the difference in specific gravity. When the OI of the halogen-free frame-retardant resin composition is 24 or more, it has a flame retardancy identical to that of PVC.